Chronic immune thrombocytopenic purpura (ITP) is a syndrome of destructive thrombocytopenia due to an antibody against a platelet-associated antigen (1,2). Van Leeuwen et al. (3) first provided evidence that autoantibodies were present in some ITP patients. They noted that of 42 antibody eluates from ITP platelets. 32 would bind to normal but not to thrombasthenic platelets; the remaining eluates bound to both. Since thrombasthenic platelets are deficient in platelet glycoproteins (GP) IIb and IIIa, they suggested that these ITP patients had autoantibodies to one of these glycoproteins. Direct evidence for anti-glycoprotein autoantibodies in chronic ITP has been provided by subsequent studies using a variety of methods. Woods et al. showed binding of autoantibodies from ITP patients to the GPIIb/IIIa complex or to GPIb attached to microtiter wells with monoclonal antibodies and confirmed these observations by immunoprecipitation (4,5). Using the former method, they noted anti-GPIIb/IIIa or anti-GPIb autoantibodies in about 10% of patients, much less than the percentage observed by the indirect studies of van Leeuwen et al. (3). Other investigators also detected autoantibodies in chronic ITP patients using immunoblotting (6,7), immunoprecipitation (4,5,8 ), inhibition of murine monoclonal anti-GPIIb/IIIa antibody binding to ITP platelets (9) and crossed immunoelectrophoresis (10). Nugent et al. (11) and Asano et al. (12) have established human hybridomas from ITP lymphocytes which synthesize monoclonal antiplatelet antibodies. Some of these are specific for platelet glycoproteins (11).
Of the arrays used for demonstrating antiglycoprotein autoantibodies in chronic ITP, the microtiter well assay (4,5) is most easily adaptable to clinical use. However, the low percentage of positive tests (about 10%) when compared to that of van Leeuwen et al. (about 76%) suggested to me that solubilization of the platelets prior to antibody sensitization may alter some of the epitopes. For this reason, I designed an assay (immunobead assay) for antiglycoprotein autoantibodies where platelets are sensitized prior to their solubilization assuming that the epitopes may remain more stable when bound to antibody. This assay can measure both platelet-associated and plasma autoantibodies. In this report, I studied 44 chronic ITP patients and noted platelet-associated autoantibodies in nine of the 11 patients studied and plasma autoantibodies in 28 of 44 patients evaluated.